The Greens have branded an initial decision to proceed with the Transmission Gully highway as short-sighted - but port bosses are applauding the infrastructure project.

Provisional consent for the hotly contested road was granted last week by a board of inquiry. The $930 million highway could be finished by 2021, with a 27kilometre road cut through the hills from Linden to rejoin State Highway 1 north of Paekakariki.

The New Zealand Transport Agency estimates it will shave 10 minutes off Kapiti peak-hour commuters' trips to and from Wellington.

The board of inquiry said the route would rectify inadequacies of the present coastal route and be safer in the event of a natural disaster.

It did, however, list environmental fallout, including an increase in sediment to Porirua Harbour, destruction of some indigenous growth, and the destruction, modification, or diversion of 10 kilometres of streams.

Plimmerton Residents' Association spokesman Colin Bleasdale agreed there were environmental concerns but he welcomed the decision, saying it would be positive for the entire region. "It gives Wellington a second escape route and that's the surety of safety that counts for us as well in case of further isolation in case of an earthquake, tsunami or otherwise."

Mr Bleasdale said he and others in the community were worried about anything that could damage the harbour, though he understood the stipulations around the route were very strict.

He expected some unforeseen problems could arise as the project progressed further.

The highway would be one of New Zealand's biggest infrastructure projects and the Wellington region's biggest roading project since the motorway extension in the late 1970s.

CentrePort chief executive Blair O'Keeffe, who supported the findings, said the decision would allow for more efficient movement of freight and people.

"We have long been a supporter of Transmission Gully and to see it making progress is very pleasing."

According to CentrePort there are about 730,000 heavy vehicles moving across the state highway at the edge of Wellington city and the numbers are continuing to grow.

But Green Party MP Jan Logie said the decision to go ahead was very disappointing and short-sighted. The mitigation of environmental damage was not the same as preventing that damage from happening in the first place.

"It's about the increase of silt going into the [Porirua] harbour, into the inlet. The community, despite there being quite a lot of audible support for this Transmission Gully, has put so much work into trying to reduce the silt levels and clean up that inlet."

The scenario was based on false projections and a better alternative would be to invest the money in public transport, Ms Logie said. She hoped to facilitate a series of community meetings to discuss options. "But it's going to be a tough road from here."

Under new national consenting processes, the Environmental Protection Authority can recommend that a board of inquiry assess applications, hear submitters and make a decision on whether projects should go ahead.

The decision is binding and can be appealed against only on points of law. Construction of the road could start in 2015 and be completed by 2021.

The authority will publish the panel's final decision in mid-June, after receiving feedback on the draft.